Thursday, February 4, 2010

Crippling Failure

Crippling is a phenomenon that occurs in a member that is under compression with sufficiently short length to prevent instability. Unstable members under compression tend to buckle as shown in figure below.

I will go over buckling some other time, but the key point to remember here is that a long member doesn’t have the lateral stability for crippling to occur because the long member will fail under buckling before crippling. Stability in aircraft are genrally provided by web and skin members. As  compressive force increases, these mating members will tend to hold the statbility of the main load carrying part. Hence allowing the part to reach crippling. For example look at the sketch below. 

In the first sketch the compressive load is evenly distributed across the entire section of the plate. As the compressive load is increase the plate will buckle and it won’t be efficient is carrying load. As I like to say “ the load is not stupid”, if the plate can nolonger carry load it will try to look for an alternative path. As you can see in Sketch 2, the load runs to the stiffer side supported by the I beams. Failure of this structure will occur when the supported sides reaches compressive yield strength of the material. This yield strength can be think of as the crippling strength.

Using the same thinking, an L angle as shown below can have it’s flanges buckle as the compressive load increases. As the compressive load further increase the load will “run” to the stiffer corner of the L-angle. Further increase in load will cause the L-angle to fail  under what we call crippling. This brings us to another key point about crippling, the crippling allowable is a function of the structures geometry. Generally, the more corner a section has, the higher the crippling allowable. For example an U-section will have an higher L-section crippling allowable as I will prove later on in another post. This post was just intended to give you an overview of what crippling is and I hope it did the job. If you have any comments, questions or correction please leave me a comment. Stay tuned for the hardcore calculations on crippling soon. 

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